The invention concerns to a method and an apparatus for zero-mixing spectrum analysis.
For example from U.S. Pat. No. 5,736,845 a spectrum analyzer with two step down conversion to the base band is known. In a first down conversion stage with a variable first local oscillator and a first mixer the input signal is transferred to an intermediate frequency. At a second stage with a constant second local oscillator and a second mixer the intermediate frequency is transferred to the base band. This configuration of spectrum analyzer is widely used. However, the two mixer stages are costly and thus it is desirable to have a spectrum analyzer with only one mixer stage. Such a concept is known as zero-mixing concept, which means that the input signal is directly converted to the base band.
The problem in frequency spectrum analyzers using the zero-mixing concept is the problem that surging occurs when the local oscillator frequency approaches the center frequency of one of the lines within the input signal. The envelope of the spectrum of the input signal has to be reconstructed. The central peak, i.e. the amplitude of the signal beyond the resolution filter when the frequency of the local oscillator equals one of the center frequencies of the lines of the input signal, strongly depends on the phase difference between the input frequency component and the sweep signal of the local oscillator. Thus no linear interpolation can reconstruct the spectrum envelope near the central peak.
In WO 03/069359 A1 it is proposed to reconstruct the envelope of the spectrum of the input signal by the use of an estimated amplitude at an estimated center frequency of each line of the input signal, whereby the estimated amplitude and the estimated center frequency are calculated from the time of occurrence, the duration and the maximum value of several halfwaves (wavelets) of the filtered base band signal. In the vicinity of the center frequency of each of the lines of the input signal, the signal outputted from the resolution filter is a surging signal which can be divided into several halfwaves (wavelets). For several halfwaves (wavelets) in the vicinity of the center frequency (when the frequency of the local oscillator approaches or leaves the center frequency) the time of occurrence, the duration and the maximum value of each halfwave are evaluated. It has been found that from the time of occurrence, the duration and the maximum value of several halfwaves an estimated center frequency and an estimated amplitude at the center frequency can be calculated. If the center frequency and the amplitude at the center frequency of each line of the input signal are known, the envelope of the input signal can be reconstructed near the center frequency.
It is a disadvantage of the method and apparatus know from WO 03/069359 A1 that the method is only performed for the real component of the complex base band signal. Thus, the number of halfwaves (wavelets) is restricted. Thus, for a sufficient accuracy of this method the sweep velocity must be limited.